The subject matter of the present invention relates generally to drainage foil apparatus for removing liquid from material carried across the foil by a moving foraminous conveyor. The liquid is drawn from the material through the conveyor and removed by the vacuum pressure and scraping action of the foil, as discussed in U.S. Pat. No. 2,928,465 of Wrist. In particular, such drainage foil apparatus is especially useful for removing the water from wood pulp stock in a paper making machine. However, the foil apparatus is also useful on other web forming machines such as those making non-woven fabrics.
At least the nose portion of the foil is made of hard crystalline ceramic material to provide a smooth wear-resistant upper surface on such nose portion which is a bearing surface engaged by the Fourdrinier wire or other foraminous conveyor. The rear portion of the foil has an upper foil surface which is spaced from the conveyor and may diverge from the plane of the bearing surface by a small angle to produce a vacuum pressure between such foil surface and the moving conveyor. The foil nose portion is made of a plurality of ceramic segments spaced laterally across the conveyor in order to reduce cost when the foil is damaged, since only the damaged segment need be replaced rather than the entire nose portion. The upper surfaces of the ceramic segments must be held in alignment with each other and the rear foil portion when it is a separate member, and accurately positioned with respect to the conveyor in order to prevent damage to the conveyor and to provide the negative pressure of the foil action. In order to provide such accurate positioning and to enable easy replacement of the ceramic segments, such segments are provided with a slot in their bottom surfaces and are bonded to a common mounting member of metal which extends into such slot. The slot and mounting member are shaped to enable each segment to be inserted on and removed from the mounting member without moving the other segments. As used herein, "Fourdrinier wire" refers to any pulp conveyor used on a paper making machine including a twin wire machine and those using synthetic plastic conveyors.
In addition, when the foil nose portion is separate from the rear portion of the foil which may be a member of a different material, such as a synthetic plastic, the ceramic nose members are resiliently urged into contact with the rear portion member by the metal mounting member to insure accurate positioning and to prevent any gaps between the bearing surface and the foil surface of such members.
The superior ceramic foil member mounting technique of the present invention not only assures accurate positioning but also enables easy replacement of damaged ceramic segments individually by unskilled personnel in the plant without stopping the paper making machine. This saves considerable time and expense which would otherwise be needed if all of the adjacent ceramic segments had to be removed before the damaged segment could be replaced, as is true of the prior art and the invention described in U.S. Pat. No. 3,870,597 of Getman et al. In this patent, ceramic nose portions are held in place by springs after the segments are slid longitudinally on and off the foil member in order to accurately position the ceramic segments relative to the conveyor wire. However, such spring mountings are not satisfactory because the different thermal expansion of the metal springs and ceramic segments causes spaces to open between the ceramic segments and between such segments and the rear foil member which will be filled with pulp and other foreign matter.
One embodiment of the above patent does show a slot in the bottom of the ceramic nose segments into which separate L-shaped metal spring members are provided. However, such springs are loosely held in a notch in the rear foil member and are not bonded to such segments so that they are subject to the above-discussed disadvantages. While such patent does disclose as prior art an all-ceramic foil having a dovetail slot in the bottom surface thereof and a dovetail mounting member apparently made of metal which extends into such slot, this foil apparatus has the disadvantage that in order to replace a single damaged ceramic segment, all of the adjacent segments would have to be removed by sliding them longitudinally along the mounting member. This requires that the foil apparatus be removed from the paper making machine and is quite time consuming. In addition, it also would prevent the ceramic segments from being bonded to the metal support. Other prior foil apparatus have attempted to solve the above problems by providing each of the ceramic nose segments with a projection on its lower surface which extends into a U-shaped metal channel mounting member or a notch in a metal support member. However, when the ceramic projection is bonded into the channel or notch, the nose member is very difficult to remove and replace among other reasons because the ceramic projection breaks off and remains within the channel or notch. The present invention overcomes these problems.
In one embodiment of the present invention employing a separate rear foil member of plastic material, either the top or bottom surfaces of such member can be employed as the foil surface which diverges from the conveyor wire by the proper angle of approximately 5.degree. or less to create a vacuum pressure which sucks the water from the pulp through the conveyor. This lengthens the life of the foil member when the foil surface becomes coated with pitch or is damaged. In addition, the top and bottom surfaces of the rear foil member can provided with different angles of divergence to change the amount of vacuum pressure created when the member is inverted in its mounting position. This is different from the foil of U.S. Pat. No. 3,165,440 of Jordansson, which changes the foil angle by reversing the position of the front and back edges of the foil rather than inverting the foil. In the patented foil, the length of the upper surfaces of the front and rear portions are different, and therefore the foil in one position is not used most efficiently. This problem is overcome by the above-discussed embodiment of the present invention.